1. Field of the Invention
The present invention relates to a technique by which a laser beam can be irradiated on a large area with a high uniformity, and also to its applied method.
2. Description of the Related Art
In recent years, there has been widely studied a technique by which an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having crystallinity such as polycrystal or microcrystal of non-single crystal) formed on an insulating substrate made of glass or the like is crystallized or improved in crystallinity by conducting a laser annealing. A silicon film is frequently used for the semiconductor film.
The glass substrate is inexpensive and rich in processing property, in comparison with a quartz substrate which has been conventionally frequently used, as a result of which it is advantageous in that a large-area substrate can be readily fabricated. This is a reason why the above study has been made. Also, the reason why the layer is frequently used for crystallization is that a melting point of the glass substrate is low. The laser can give a high energy to only the non-single crystal film without largely changing the temperature of the substrate.
Because a crystalline silicon film formed by laser annealing is high in mobility, it is extensively utilized for a monolithic liquid crystal electro-optic device, etc., where a thin film transistor (TFT) is formed using the crystalline silicon film, for example, to fabricate TFTs for pixel and a drive circuit on a single glass substrate. Because the crystalline silicon film is made of a large number of crystal grains, it is called "polycrystal silicon film" or "polycrystal semiconductor film".
Also, a method in which a pulse laser beam having a large output such as excimer laser or the like is processed through an optical system so as to form a square spot of several cm square or a line of several mm width x several 10 cm on a plane to be irradiated, and the laser beam is scanned (while a position to which the laser beam is irradiated is moved relatively with respect to the plane to be irradiated) to conduct laser annealing has been frequently used because it is high in productivity and excellent from the industrial viewpoint.
In particular, the use of the linear laser beam is different from the use of a spot-like laser beam requiring scanning in the front and rear direction and in the right and left direction in that the laser can be irradiated on the entire plane to be irradiated by scanning only in a direction (width direction) perpendicular to the linear direction (longitudinal direction) of the linear laser, the high productivity can be obtained. The reason that scanning is made in the direction perpendicular to the linear direction is because it is the scanning direction where the coefficient is the highest. Because of the high productivity, the linear laser beam is being mainly used for laser annealing at present.
However, there arise some problems when laser annealing is conducted on the non-single crystal semiconductor film while scanning a pulse laser beam which has been processed into a line. In particular, one of the severe problems is that laser annealing is not uniformly conducted on the entire film surface. When the linear laser started to be used, a phenomenon that stripes are produced on portions where the adjacent beams are overlapped with each other was remarkable, and the semiconductor characteristic of the film was largely different depending on each of the stripes.
What is shown in FIG. 1A is a state of the stripes. The stripes are exhibited by the amount of reflection of a light when the surface of the silicon film after being annealed by a laser is observed.
In case of FIG. 1A, KrF excimer laser is used as a linear laser beam that extends in the right and left direction of a paper surface, and is irradiated while being scanned from the upper of the paper toward the lower thereof.
It is understood that the lateral stripes of FIG. 1A is caused by the overlapped degree of the pulse laser shots.
In the case of fabricating the active matrix liquid crystal display device using a silicon film exhibiting the stripped pattern shown in FIG. 1A there occurs a disadvantage that the stripes appear as they are.
This problem is being improved by improving the non-single crystal semiconductor film which is an object onto which a laser is irradiated, or thinning the scanning pitches (intervals between the adjacent linear laser beams) of the linear laser.
However, subsequent to the solving of the problem caused by the overlapped pulse laser shots, the nonuniformity of the energy distribution of the beam per se has been remarkable.
In general, in the case of forming the linear laser beam, an original rectangular beam is processed into a line through an appropriate optical system. The original rectangular beam is about 2 to 5 in aspect ratio, and for example, the original beam is deformed into a linear beam 100 or more in aspect ratio through an optical system shown in FIG. 2. The optical system is designed so as to unify the distribution within the beam of energy at the same time.
The device shown in FIG. 2 has a function to irradiate a laser beam emitted from an oscillator 201 (which is in the form of substantially a square in this state) as linear beams through an optical system indicated by reference numerals 202, 203, 204, 205 and 207. Reference numeral 206 denotes a mirror.
What is denoted by reference numeral 203 is called "a cylindrical lens group" (which is also called "a multiple cylindrical lens" or "a flyeye lens") and has a function to divide the beams into a large number of beams. The large number of beams as divided is re-synthesized by the cylindrical lens 205.
This structure is required to improve the distribution of intensity within the beam. Also, the combination of the cylindrical lens group 202 with the cylindrical lens 204 has the same function as the above-described combination of the cylindrical lens group 203 with the cylindrical lens 205.
In other words, the combination of the cylindrical lens group 203 with the cylindrical lens 205 has a function to improve the distribution of intensity of the linear laser beam in its longitudinal direction, and the combination of the cylindrical lens group 202 with the cylindrical lens 204 has a function to improve the distribution of intensity of the linear laser beam in its width direction.
The optical system designed to unify the distribution of an energy within the beam is called "a beam homogenizer". The optical system shown in FIG. 2 is also one of the beam homogenizers. The method of unifying the distribution of an energy is that after the original rectangular beam is divided, the respective divided beams are enlarged and superimposed on each other to unify the distribution of an energy.
Seemingly, in the beam which is divided and re-constructed in the above method, the distribution of the energy becomes uniform more as the division of the beam becomes more fine. However, in fact, when the beam is irradiated onto the semiconductor film, the stripped pattern shown in FIG. 1B appears regardless of the fineness of the division.
The irradiation of a laser onto a silicon film shown in FIG. 1B is also an example in which a linear KrF excimer laser beam extending in the right and left direction on the paper is scanned from the upper of the paper toward the lower thereof for irradiation as in the case of FIG. 1A. Here, the laser was irradiated under the scanning conditions where the stripes caused by the overlapping of the laser shot shown in FIG. 1A are not remarkably exhibited.
The stripped patterns are innumerably formed so as to be orthogonal to the longitudinal direction of the linear laser beam. The formation of such stripped pattern is caused by the stripped distribution of the energy of the original rectangular beam or optical system.